In recent years, the reduction of CO2 emissions has been sincerely desired in order to address air pollution and global warming. The automotive industry has a growing expectation on the introduction of electric vehicles (EV) and hybrid electric vehicles (HEV) for the reduction of CO2 emissions and has been intensively working on the development of motor-driving secondary batteries, which become key to the practical application of these electric vehicles.
The motor-driving secondary batteries are required to have very high output performance and high energy as compared to consumer lithium-ion secondary batteries for mobile phones, notebook computers etc. Among all batteries, attentions are being given to lithium-ion secondary batteries having relatively high theoretical energy. The development of the lithium-ion secondary batteries has been pursued rapidly at present.
In general, the lithium-ion secondary battery includes a positive electrode in which a positive electrode active material is applied to both sides of a positive electrode collector with the use of a binder etc. and a negative electrode in which a negative electrode active material is applied to both sides of a negative electrode collector with the use of a binder etc. The positive and negative electrodes are connected to each other via an electrolyte layer and accommodated in a battery case.
Conventionally, carbon/graphite materials are used for negative electrodes of lithium-ion secondary batteries in terms of charge/discharge cycle lifetime and cost advantage. However, the carbon/graphite-based negative electrode active materials perform charge/discharge operation by absorption and desorption of lithium ions to and from graphite crystals and thus have the drawback that these negative electrode active materials cannot obtain a charge/discharge capacity higher than or equal to 372 mAh/g, that is, the theoretical capacity of the maximum lithium intercalation compound LiC6. It is difficult for the carbon/graphite-based negative electrode active materials to secure a satisfactory level of capacity and energy density for practical use in vehicles.
On the other hand, materials capable of alloying with Li are expected as negative electrode materials for vehicle uses due to the fact that batteries using these Li alloying materials improve in energy density as compared to those using conventional carbon/graphite-based negative electrode active materials. In the case of Si material, for example, there occurs absorption and desorption of 4.4 mol of lithium ions per 1 mol of Si during charge/discharge operation as indicated in the following reaction scheme (1). The theoretical capacity of Li22Si5 (═Li4.4Si) reaches 2100 mAh/g. The Si material has an initial capacity 3200 mAh/g per weight.Si+4.4Li++e−Li4.4Si  (1)
In the lithium-ion secondary battery, however, the Li-alloying negative electrode material shows a high degree of expansion and contraction during charge/discharge operation. While the graphite material expands about 1.2 times in volume by absorption of Li ions, the Si material shows a larger volume change (i.e. expands about 4 times in volume) by transition from amorphous to crystalline phase during alloying of Si with Li. This results in a deterioration of electrode cycle lifetime. Further, the Si material has a trade-off relationship between capacity and cycle durability so that it is difficult to improve the cycle durability of the Si material while securing the high capacity of the Si material.
In order to solve the above problems, there has been proposed “alloying” of Si by addition of various metal elements” as the technique for improving the cycle lifetime of Si as the negative electrode active material. However, many proposals (such as inventions) about alloying of Si relate to composite materials in which Si is mixed with other metal elements. Many of these Si composite materials become much lower in capacity than Si. These Si composite materials also significantly decrease in initial charge/discharge efficiency with increase in secondary phase content. Further, it is known that the alloyed Si negative electrode active materials decrease in initial charge/discharge efficiency with increase in metal doping concentration. For example, there has been proposed a negative electrode active material for a lithium-ion secondary battery containing an amorphous alloy having a composition represented by the formula: SixMyAlz (see, for example, Patent Document 1). In the formula, x, y and z represent atomic percent values and satisfy the following conditions: x+y+z=100, x≧55, y<22 and z>0; and M represents at least one metal selected from the group consisting of Mn, Mo, Nb, W, Ta, Fe, Cu, Ti, V, Cr, Ni, Co, Zr and Y. It is described in paragraph [0018] of Patent Document 1 that the amorphous alloy-containing negative electrode active material can attain not only a high capacity but also good cycle lifetime by minimizing the content of the metal M.
By the use of the amorphous alloy SixMyAlz of Patent Document 1 in the negative electrode, it may be possible for the lithium-ion secondary battery to attain high cycle performance. Even in this case, however, the initial capacity and cycle capacity of the lithium-ion secondary battery are not at a sufficiently high level.
As mentioned above, the electrodes using the alloyed Si negative electrode active materials of Patent Document 1 etc. and batteries using the same make improvements in cycle durability but face the problem of decreases in effective battery capacity due to low initial charge/discharge efficiency. In addition, very difficult adjustments are required in actual battery production due to increase in difference between positive and negative electrode characteristics. Although the “charge/discharge capacity” and “cycle durability” are specifically noted as the performance required of the negative electrode active material, not only the “charge/discharge capacity” and “cycle durability” but also the “initial charge/discharge efficiency” are very important parameters for the actual battery applications. However, there has been no negative electrode active material that attains good balance between these electrochemical characteristics.